Crucibles (for instance made of fused silica, silicon carbide, quartz, silicon nitride, reaction bonded silicon nitride, or graphite) are typically used in solidification of polycrystalline silicon. Silica is chosen primarily for high purity and availability. There are problems in using silica, however, as a crucible for the production of silicon by this method.
Silicon in its molten state will react with the silica crucible that is in contact with it. Molten silicon reacts with silica to form silicon monoxide and oxygen. Oxygen will contaminate the silicon. Silicon monoxide is volatile, and will react with the graphite components inside the furnace. Silicon monoxide reacts with graphite to form silicon carbide and carbon monoxide. The carbon monoxide will then react with the molten silicon, forming additional volatile silicon monoxide, silicon carbide, carbides and oxides of metallic traces or additives and carbon. Carbon will contaminate the silicon. Silicon can also react with the various impurities contained in the crucible (iron, boron, aluminum, . . . ) and/or contained in the nitride coating.
The reaction between silica and silicon promotes adhesion of the silicon to the crucible. This adhesion, combined with a difference in coefficients of thermal expansion between the two materials, creates stress in the silicon ingot, causing it to crack on cooling. It is known in the art that a protective coating applied to the inside of the crucible in the area of contact with the ingot can prevent the reaction between silicon and silica that leads to ingot contamination and cracking. To be effective, the coating must be thick enough to prevent the silicon from reacting with the silica crucible, and must not adversely contaminate the silicon either by itself or from contaminants within it.
A variety of materials and techniques are described in the literature, which attempt to solve the problem of reaction and adhesion of the crucible in contact with molten material.
Silicon nitride coatings are known to prevent the chemical reaction between molten silicon and silica from the crucible. U.S. Pat. No. 4,741,925 describes a silicon nitride coating for crucibles applied by chemical vapor deposition at 1250° C. while WO-A1-2004/053207 discloses a silicon nitride coating applied by plasma spraying. U.S. Pat. No. 4,218,418 describes a technique of forming a glass layer inside a silica crucible by rapid heating to prevent cracking of silicon during melt-processing.
Prior art includes specific references to powdered mold release agents for application to crucibles in the directional solidification of silicon. In addition, the use of chemical vapor deposition, solvent evaporation, high-temperature flame treatment, and other expensive and complex means are mentioned for application of crucible coatings. References are made to specific binders and solvents. References are made to mixing, spraying, or brushing for slurries of powdered coatings.
Silicon nitride coatings are known to prevent the chemical reaction between molten silicon and silica from the crucible.
However, the silicon nitride coating itself can lead to problems. The thickness of the silicon nitride coating necessary to prevent the silicon from reacting with the silica crucible is quite important (about 300 μm) making thereby the coating operation expensive and time consuming. Further, this silicon nitride coating is mechanically weak and can peel or flake off during or even before use. It is therefore recommended to apply this coating at the very last moment before use, i.e., at the end user facilities, leaving thereby the burden of applying this thick coating to the end user.
The known technologies to provide a stable nitride coating onto a ceramic crucible include (1) the oxidation of the nitride coating at high temperature ranging from 700° C. to 1450° C. under a controlled burnout cycle and (2) the addition of sintering/sticking (or adherence) aids to the nitride composition. Additives can be metals or oxides additives such as Al2O3, SiO2, AlN, Al, Si, fume or fine silica and others. A silicon nitride coating comprising fume silica is described in the co-pending application EP04447105. The oxidation of the silicon nitride into silicon oxide increases the quantity of oxygen in the coating and leads to the problem mentioned above. In addition the level of oxidation and resulting amount of oxygen is not easy to control.
The need to maintain low oxygen content in the crucible coating was highlighted by most of the publications of silicon producers describing the chemical and physical interactions during photovoltaic and semi-conductor application. The use of low oxygen silicon nitride coating is recommended for high quality wafer production. The use of high purity silicon nitride powder with low oxygen content has been described notably in U.S. Pat. No. 6,165,425. This document describes a silicon nitride coating which has an extremely low oxygen content ranging from 0.3% to at most 5% by weight. The coating can comprise adhesion promoters such as polyvinyl alcohol and is dried in air at a temperature preferably ranging from 500° to 700° C. At these low drying temperatures, the oxidation of the silicon nitride does not take place, there is no formation of SiO2 on the grains boundary and the full effectiveness of the silicon nitride is kept. However, some problems remain. As there is no oxidation of the coating, the coating remains pulverulent and is easily damaged when liquid silicon is charged into the crucible.